I have been to Belize once and Roatan twice. I would not go back to Belize unless I were staying at one of the atolls. I am not sure where people who would choose San Pedro/Ambergris diving over Roatan dove in Belize, but it wasn't the same place we dove. My experience was that Belize (Ambergris Caye area) was the cheaper of the two and the diving was not anywhere near as good as Roatan. The diving is quite good at the atolls in Belize, probably equal to or a little better than Roatan, but if you stay near San Pedro for the nightlife, you probably won't get to dive the atolls more than once or if lucky twice in the week, and you will pay a big premium for the trip. Also, staying at the atolls is much more expensive. The resorts on the atolls themselves are pretty small and quiet, and probably wouldn't meet the nightlife requirement anyways.
I would go with someplace on Roatan near West End...possible Inn of Last Resort or one of the smaller West End lodging places, and then pick an independent operator for your dives. You can get some amazingly cheap lodging options in West End if you aren't looking for luxury, and West End is probably the best location for restaurants and bars on Roatan. The resorts like Anthony's Key, CocoView, and Fantasy Island have bars, but most of them are closed around 10pm and may or may not be very hopping depending on who is there at the resort that week.
Good Luck and have fun!
AggieDiver